1. Field of the Invention
The present invention relates to a photoconductive drum unit which defines an electrophotographic image forming device. The image forming device is used in a facsimile machine, a copier, or a printer or the like (including a Multi Function Peripheral (MFP) of the facsimile machine, the copier, and/or the printer). More specifically, the present invention relates to a photoconductive drum unit including a scorotron charging unit.
2. Description of the Related Art
In the electrophotographic image forming device, a photoconductive drum is rotatably supported on a unit frame. A scorotron charging unit is mounted and fixed on the unit frame such that the scorotron charging unit is arranged to face a surface of the photoconductive drum and along a longitudinal direction of the photoconductive drum. The unitized photoconductive drum unit is widely used. In such a scorotron charging method, when a distance between the surface of the photoconductive drum and a grid is not maintained at an equal distance at all positions in the longitudinal direction of the photoconductive drum, an uneven charge is generated in the longitudinal direction of the photoconductive drum which influences the image quality.
In a conventional charging unit, a shield casing, a discharge wire (filament), and a grid are integrally assembled as one unit. The charging unit is fixed on a unit frame by tightening screws at both ends of the unit frame in its longitudinal direction. Further, the unit frame supports a photoconductive drum. Since the unit frame is made of resin or plastic, the unit frame may be twisted or distorted in its longitudinal direction. Additionally, since the charging unit is formed as a solid singular body, when the charging unit is fixed onto the unit frame by tightening the screws, it is difficult for the charging unit to respond to the distortion of the unit frame. Thus, there has been a problem that a distance between the grid and the surface of the photoconductive drum is not constant in the longitudinal direction.
According to one conventional device, a scorotron charging unit includes a holding member that holds a grid plate on a shaft of a photoconductor. In such a charging unit, there has been a problem that a distance between the surface of the photoconductor and the grid plate changes according to a degree of eccentricity of the shaft. Therefore, according to a conventionally known scorotron charging unit, a positioning member is provided for maintaining a prescribed distance between the surface of the photoconductor and the grid plate by making contact with the surface of the photoconductor. The positioning member applies tension to the grid plate in its longitudinal direction. According to another conventional device, a grid electrode is provided separately from a main body of a charging device. The grid electrode is supported via a positioning member and a position adjusting device with respect to a photoconductor supporting body (unit frame), which rotatably supports a photoconductive drum.
In a conventional scorotron charging unit, the charging unit is supported between holders that are fixedly mounted on the unit frame. Accordingly, since the holding members of the grid plate are positioned by the positioning member making contact with the surface of the photoconductor, a prescribed interval can be maintained between the grid plate and the photoconductor. However, since the positioning member is required to be coated with a low-friction material such as TEFLON (registered trademark), an increase in cost is inevitable. Moreover, since the holders fixedly mounted on the unit frame are integrally formed with the charging unit, the charging unit and the unit frame are formed as one solid body. Therefore, even when the unit frame is distorted, it is difficult for the charging unit to respond to such distortion. As a result, it is also difficult to reliably mount and fix the charging unit in such a situation.
According to a second known charging device, a grid electrode is provided separately from a charging device main body, and is mounted on a unit frame of a photoconductive drum. According to this structure, since a positioning member and a position adjusting device are required, the number of components increases causing an increase in cost. Furthermore, since the grid electrode is mounted on the unit frame, when the charging device and the photoconductive drum are formed as a drum unit, it becomes necessary to adjust the position of the charging device main body and the grid electrode.